The present invention relates to means for the detection and characterization of nucleic acid sequences and variations in nucleic acid sequences. The present invention relates to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site-specific manner. The 5xe2x80x2 nuclease activity of a variety of enzymes is used to cleave the target-dependent cleavage structure, thereby indicating the presence of specific nucleic acid sequences or specific variations thereof. The present invention further provides novel methods and devices for the separation of nucleic acid molecules based by charge.
The detection and characterization of specific nucleic acid sequences and sequence variations has been utilized to detect the presence of viral or bacterial nucleic acid sequences indicative of an infection, the presence of variants or alleles of mammalian genes associated with disease and cancers and the identification of the source of nucleic acids found in forensic samples, as well as in paternity determinations.
Various methods are known to the art which may be used to detect and characterize specific nucleic acid sequences and sequence variants. Nonetheless, as nucleic acid sequence data of the human genome, as well as the genomes of pathogenic organisms accumulates, the demand for fast, reliable, cost-effective and user-friendly tests for the detection of specific nucleic acid sequences continues to grow. Importantly, these tests must be able to create a detectable signal from samples which contain very few copies of the sequence of interest. The following discussion examines two levels of nucleic acid detection assays currently in use: I. Signal Amplification Technology for detection of rare sequences; and II. Direct Detection Technology for detection of higher copy number sequences.
I. Signal Amplification Technology Methods for Amplification
The xe2x80x9cPolymerase Chain Reactionxe2x80x9d (PCR) comprises the first generation of methods for nucleic acid amplification. However, several other methods have been developed that employ the same basis of specificity, but create signal by different amplification mechanisms. These methods include the xe2x80x9cLigase Chain Reactionxe2x80x9d (LCR), xe2x80x9cSelf-Sustained Synthetic Reactionxe2x80x9d (3SR/NASBA), and xe2x80x9cQxcex2-Replicasexe2x80x9d (Qxcex2).
Polymerase Chain Reaction (PCR)
The polymerase chain reaction (PCR), as described in U.S. Pat. Nos. 4,683,195 and 4,683,202 to Mullis and Mullis et al. (the disclosures of which are hereby incorporated by reference), describe a method for increasing the concentration of a segment of target sequence in a mixture of genomic DNA without cloning or purification. This technology provides one approach to the problems of low target sequence concentration. PCR can be used to directly increase the concentration of the target to an easily detectable level. This process for amplifying the target sequence involves introducing a molar excess of two oligonucleotide primers which are complementary to their respective strands of the double-stranded target sequence to the DNA mixture containing the desired target sequence. The mixture is denatured and then allowed to hybridize. Following hybridization, the primers are extended with polymerase so as to form complementary strands. The steps of denaturation, hybridization, and polymerase extension can be repeated as often as needed, in order to obtain relatively high concentrations of a segment of the desired target sequence.
The length of the segment of the desired target sequence is determined by the relative positions of the primers with respect to each other, and, therefore, this length is a controllable parameter. Because the desired segments of the target sequence become the dominant sequences (in terms of concentration) in the mixture, they are said to be xe2x80x9cPCR-amplified.xe2x80x9d
Ligase Chain Reaction (LCR or LAR)
The ligase chain reaction (LCR; sometimes referred to as xe2x80x9cLigase Amplification Reactionxe2x80x9d (LAR) described by Barany, Proc. Natl. Acad. Sci., 88:189 (1991); Barany, PCR Methods and Applic., 1:5 (1991); and Wu and Wallace, Genomics 4:560 (1989) has developed into a well-recognized alternative method for amplifying nucleic acids. In LCR, four oligonucleotides, two adjacent oligonucleotides which uniquely hybridize to one strand of target DNA, and a complementary set of adjacent oligonucleotides, which hybridize to the opposite strand are mixed and DNA ligase is added to the mixture. Provided that there is complete complementarity at the junction, ligase will covalently link each set of hybridized molecules. Importantly, in LCR, two probes are ligated together only when they base-pair with sequences in the target sample, without gaps or mismatches. Repeated cycles of denaturation, hybridization and ligation amplify a short segment of DNA. LCR has also been used in combination with PCR to achieve enhanced detection of single-base changes. Segev, PCT Public. No. W09001069 A1 (1990). However, because the four oligonucleotides used in this assay can pair to form two short ligatable fragments, there is the potential for the generation of target-independent background signal. The use of LCR for mutant screening is limited to the examination of specific nucleic acid positions.
Self-Sustained Synthetic Reaction (3SR/NASBA)
The self-sustained sequence replication reaction (3SR) (Guatelli et al., Proc. Natl. Acad. Sci., 87:1874-1878 [1990], with an erratum at Proc. Natl. Acad. Sci., 87:7797 [1990]) is a transcription-based in vitro amplification system (Kwok et al., Proc. Natl. Acad. Sci., 86:1173-1177 [1989]) that can exponentially amplify RNA sequences at a uniform temperature. The amplified RNA can then be utilized for mutation detection (Fahy et al., PCR Meth. Appl., 1:25-33 [1991]). In this method, an oligonucleotide primer is used to add a phage RNA polymerase promoter to the 5xe2x80x2 end of the sequence of interest. In a cocktail of enzymes and substrates that includes a second primer, reverse transcriptase, RNase H, RNA polymerase and ribo-and deoxyribonucleoside triphosphates, the target sequence undergoes repeated rounds of transcription, cDNA synthesis and second-strand synthesis to amplify the area of interest. The use of 3SR to detect mutations is kinetically limited to screening small segments of DNA (e.g., 200-300 base pairs).
Q-Beta (Qxcex2) Replicase
In this method, a probe which recognizes the sequence of interest is attached to the replicatable RNA template for Qxcex2 replicase. A previously identified major problem with false positives resulting from the replication of unhybridized probes has been addressed through use of a sequence-specific ligation step. However, available thermostable DNA ligases are not effective on this RNA substrate, so the ligation must be performed by T4 DNA ligase at low temperatures (37xc2x0 C.). This prevents the use of high temperature as a means of achieving specificity as in the LCR, the ligation event can be used to detect a mutation at the junction site, but not elsewhere.
Table 1 below, lists some of the features desirable for systems useful in sensitive nucleic acid diagnostics, and summarizes the abilities of each of the major amplification methods (See also, Landgren, Trends in Genetics 9:199 [1993]).
A successful diagnostic method must be very specific. A straight-forward method of controlling the specificity of nucleic acid hybridization is by controlling the temperature of the reaction. While the 3SR/NASBA, and Qxcex2 systems are all able to generate a large quantity of signal, one or more of the enzymes involved in each cannot be used at high temperature (i.e.,  greater than 55xc2x0 C.). Therefore the reaction temperatures cannot be raised to prevent non-specific hybridization of the probes. If probes are shortened in order to make them melt more easily at low temperatures, the likelihood of having more than one perfect match in a complex genome increases. For these reasons, PCR and LCR currently dominate the research field in detection technologies.
The basis of the amplification procedure in the PCR and LCR is the fact that the products of one cycle become usable templates in all subsequent cycles, consequently doubling the population with each cycle. The final yield of any such doubling system can be expressed as: (1+X)n=y, where xe2x80x9cXxe2x80x9d is the mean efficiency (percent copied in each cycle), xe2x80x9cnxe2x80x9d is the number of cycles, and xe2x80x9cyxe2x80x9d is the overall efficiency, or yield of the reaction (Mullis, PCR Methods Applic., 1:1 [1991]). If every copy of a target DNA is utilized as a template in every cycle of a polymerase chain reaction, then the mean efficiency is 100%. If 20 cycles of PCR are performed, then the yield will be 220, or 1,048,576 copies of the starting material. If the reaction conditions reduce the mean efficiency to 85%, then the yield in those 20 cycles will be only 1.8520, or 220,513 copies of the starting material. In other words, a PCR running at 85% efficiency will yield only 21% as much final product, compared to a reaction running at 100% efficiency. A reaction that is reduced to 50% mean efficiency will yield less than 1% of the possible product.
In practice, routine polymerase chain reactions rarely achieve the theoretical maximum yield, and PCRs are usually run for more than 20 cycles to compensate for the lower yield. At 50% mean efficiency, it would take 34 cycles to achieve the million-fold amplification theoretically possible in 20, and at lower efficiencies, the number of cycles required becomes prohibitive. In addition, any background products that amplify with a better mean efficiency than the intended target will become the dominant products.
Also, many variables can influence the mean efficiency of PCR, including target DNA length and secondary structure, primer length and design, primer and dNTP concentrations, and buffer composition, to name but a few. Contamination of the reaction with exogenous DNA (e.g., DNA spilled onto lab surfaces) or cross-contamination is also a major consideration. Reaction conditions must be carefully optimized for each different primer pair and target sequence, and the process can take days, even for an experienced investigator. The laboriousness of this process, including numerous technical considerations and other factors, presents a significant drawback to using PCR in the clinical setting. Indeed, PCR has yet to penetrate the clinical market in a significant way. The same concerns arise with LCR, as LCR must also be optimized to use different oligonucleotide sequences for each target sequence. In addition, both methods require expensive equipment, capable of precise temperature cycling.
Many applications of nucleic acid detection technologies, such as in studies of allelic variation, involve not only detection of a specific sequence in a complex background, but also the discrimination between sequences with few, or single, nucleotide differences. One method for the detection of allele-specific variants by PCR is based upon the fact that it is difficult for Taq polymerase to synthesize a DNA strand when there is a mismatch between the template strand and the 3xe2x80x2 end of the primer. An allele-specific variant may be detected by the use of a primer that is perfectly matched with only one of the possible alleles; the mismatch to the other allele acts to prevent the extension of the primer, thereby preventing the amplification of that sequence. This method has a substantial limitation in that the base composition of the mismatch influences the ability to prevent extension across the mismatch, and certain mismatches do not prevent extension or have only a minimal effect (Kwok et al., Nucl. Acids Res., 18:999 [1990]).) A similar 3xe2x80x2-mismatch strategy is used with greater effect to prevent ligation in the LCR (Barany, PCR Meth. Applic., 1:5 [1991]). Any mismatch effectively blocks the action of the thermostable ligase, but LCR still has the drawback of target-independent background ligation products initiating the amplification. Moreover, the combination of PCR with subsequent LCR to identify the nucleotides at individual positions is also a clearly cumbersome proposition for the clinical laboratory.
H. Direct Detection Technology
When a sufficient amount of a nucleic acid to be detected is available, there are advantages to detecting that sequence directly, instead of making more copies of that target, (e.g., as in PCR and LCR). Most notably, a method that does not amplify the signal exponentially is more amenable to quantitative analysis. Even if the signal is enhanced by attaching multiple dyes to a single oligonucleotide, the correlation between the final signal intensity and amount of target is direct. Such a system has an additional advantage that the products of the reaction will not themselves promote further reaction, so contamination of lab surfaces by the products is not as much of a concern. Traditional methods of direct detection including Northern and Southern blotting and RNase protection assays usually require the use of radioactivity and are not amenable to automation. Recently devised techniques have sought to eliminate the use of radioactivity and/or improve the sensitivity in automatable formats. Two examples are the xe2x80x9cCycling Probe Reactionxe2x80x9d (CPR), and xe2x80x9cBranched DNAxe2x80x9d (bDNA)
The cycling probe reaction (CPR) (Duck et al., BioTech., 9:142 [1990]), uses a long chimeric oligonucleotide in which a central portion is made of RNA while the two termini are made of DNA. Hybridization of the probe to a target DNA and exposure to a thermostable RNase H causes the RNA portion to be digested. This destabilizes the remaining DNA portions of the duplex, releasing the remainder of the probe from the target DNA and allowing another probe molecule to repeat the process. The signal, in the form of cleaved probe molecules, accumulates at a linear rate. While the repeating process increases the signal, the RNA portion of the oligonucleotide is vulnerable to RNases that may carried through sample preparation.
Branched DNA (bDNA), described by Urdea et al., Gene 61:253-264 (1987), involves oligonucleotides with branched structures that allow each individual oligonucleotide to carry 35 to 40 labels (e.g., alkaline phosphatase enzymes). While this enhances the signal from a hybridization event, signal from non-specific binding is similarly increased.
While both of these methods have the advantages of direct detection discussed above, neither the CPR or bDNA methods can make use of the specificity allowed by the requirement of independent recognition by two or more probe (oligonucleotide) sequences, as is common in the signal amplification methods described in section I. above. The requirement that two oligonucleotides must hybridize to a target nucleic acid in order for a detectable signal to be generated confers an extra measure of stringency on any detection assay. Requiring two oligonucleotides to bind to a target nucleic acid reduces the chance that false xe2x80x9cpositivexe2x80x9d results will be produced due to the non-specific binding of a probe to the target. The further requirement that the two oligonucleotides must bind in a specific orientation relative to the target,as is required in PCR, where oligonucleotides must be oppositely but appropriately oriented such that the DNA polymerase can bridge the gap between the two oligonucleotides in both directions, further enhances specificity of the detection reaction. However, it is well known to those in the art that even though PCR utilizes two oligonucleotide probes (termed primers) xe2x80x9cnon-specificxe2x80x9d amplification (i.e., amplification of sequences not directed by the two primers used) is a common artifact. This is in part because the DNA polymerase used in PCR can accommodate very large distances, measured in nucleotides, between the oligonucleotides and thus there is a large window in which non-specific binding of an oligonucleotide can lead to exponential amplification of inappropriate product. The LCR, in contrast, cannot proceed unless the oligonucleotides used are bound to the target adjacent to each other and so the full benefit of the dual oligonucleotide hybridization is realized.
An ideal direct detection method would combine the advantages of the direct detection assays (e.g., easy quantification and minimal risk of carry-over contamination) with the specificity provided by a dual oligonucleotide hybridization assay.
The present invention relates to means for cleaving a nucleic acid cleavage structure in a site-specific manner. In one embodiment, the means for cleaving is a cleaving enzyme comprising 5xe2x80x2 nucleases derived from thermostable DNA polymerases. These polymerases form the basis of a novel method of detection of specific nucleic acid sequences. The present invention contemplates use of novel detection methods for various uses, including, but not limited to clinical diagnostic purposes.
In one embodiment, the present invention contemplates a DNA sequence encoding a DNA polymerase altered in sequence (ie., a xe2x80x9cmutantxe2x80x9d DNA polymerase) relative to the native sequence, such that it exhibits altered DNA synthetic activity from that of the native (i.e., xe2x80x9cwild typexe2x80x9d) DNA polymerase. It is preferred that the encoded DNA polymerase is altered such that it exhibits reduced synthetic activity compared to that of the native DNA polymerase. In this manner, the enzymes of the invention are predominantly 5xe2x80x2 nucleases and are capable of cleaving nucleic acids in a structure-specific manner in the absence of interfering synthetic activity.
Importantly, the 5xe2x80x2 nucleases of the present invention are capable of cleaving linear duplex structures to create single discrete cleavage products. These linear structures are either 1) not cleaved by the wild type enzymes (to any significant degree), or 2) are cleaved by the wild type enzymes so as to create multiple products. This characteristic of the 5xe2x80x2 nucleases has been found to be a consistent property of enzymes derived in this manner from thermostable polymerases across eubacterial thermophilic species.
It is not intended that the invention be limited by the nature of the alteration necessary to render the polymerase synthesis-deficient. Nor is it intended that the invention be limited by the extent of the deficiency. The present invention contemplates various structures, including altered structures (primary, secondary, etc.), as well as native structures, that may be inhibited by synthesis inhibitors.
Where the polymerase structure is altered, it is not intended that the invention be limited by the means by which the structure is altered. In one embodiment, the alteration of the native DNA sequence comprises a change in a single nucleotide. In another embodiment, the alteration of the native DNA sequence comprises a deletion of one or more nucleotides. In yet another embodiment, the alteration of the native DNA sequence comprises an insertion of one or more nucleotides. It is contemplated that the change in DNA sequence may manifest itself as change in amino acid sequence.
The present invention contemplates 5xe2x80x2 nucleases from a variety of sources, including mesophilic, psychrophilic, thermophilic, and hyperthermophilic organisms. The preferred 5xe2x80x2 nucleases are thermostable. Thermostable 5xe2x80x2 nucleases are contemplated as particularly useful in that they operate at temperatures where nucleic acid hybridization is extremely specific, allowing for allele-specific detection (including single-base mismatches). In one embodiment, the thermostable 5xe2x80x2 nucleases are selected from the group consisting of altered polymerases derived from the native polymerases of Thermus species, including, but not limited to Thermus aquaticus, Thermus flavus, and Thermus thermophilus. However, the invention is not limited to the use of thermostable 5xe2x80x2 nucleases.
As noted above, the present invention contemplates the use of altered polymerases in a detection method. In one embodiment, the present invention provides a method of detecting the presence of a target RNA by detecting non-target cleavage products comprising: a) providing: i) a cleavage means, ii) a source of target RNA, where the target RNA has a first region, a second region and a third region, wherein the first region is located adjacent to and downstream from the second region, and the second region is located adjacent to and downstream from the third region, iii) a first oligonucleotide having a 5xe2x80x2 and a 3xe2x80x2 portion, wherein the 5xe2x80x2 portion of the first oligonucleotide contains a sequence complementary to the second region of the target RNA and wherein the 3xe2x80x2 portion of the first oligonucleotide contains a sequence complementary to the third region of the target RNA, iv) a second oligonucleotide having a 5xe2x80x2 and a 3xe2x80x2 portion wherein the 5xe2x80x2 portion of the second oligonucleotide contains a sequence complementary to the first region of the target RNA, and the 3xe2x80x2 portion of the second oligonucleotide contains a sequence complementary to the second region of the target RNA; b) mixing the cleavage means, the target RNA, and the first and second oligonucleotides, to create a reaction mixture under reaction conditions such that at least the 3xe2x80x2 portion of the first oligonucleotide is annealed to the target RNA, and wherein at least the 5xe2x80x2 portion of the second oligonucleotide is annealed to the target RNA so as to create a cleavage structure, and wherein cleavage of the cleavage structure occurs to generate non-target cleavage products; and c) detecting the non-target cleavage products.
It is contemplated that the first, second and third regions of the target be located adjacent to each other. However, the invention is not limited to the use of a target in which the three regions are contiguous with each other. Thus, the present invention contemplates the use of target RNAs wherein these three regions are contiguous with each other, as well as target RNAs wherein these three regions are not contiguous. It is further contemplated that gaps of approximately 2-10 nucleotides, representing regions of non-complementarity to the oligonucleotides (e.g., the first and/or second oligonucleotides), may be present between the three regions of the target RNA.
In at least one embodiment, it is intended that mixing of step b) is conducted under conditions such that at least the 3xe2x80x2 portion of the first oligonucleotide is annealed to the target RNA, and wherein at least the 5xe2x80x2 portion of the second oligonucleotide is annealed to the target RNA. In this manner a cleavage structure is created and cleavage of this cleavage structure can occur. These conditions allow for the use of various formats. In a preferred format, the conditions of mixing comprises mixing together the target RNA with the first and second oligonucleotides and the cleavage means in an aqueous solution in which a source of divalent cations is lacking. In this format, the cleavage reaction is initiated by the addition of a solution containing Mn2+ or Mg2+ ions. In another preferred format, the conditions of mixing comprises mixing together the target RNA, and the first and second oligonucleotides in an aqueous solution containing Mn2+ or Mg2+ ions, and then adding the cleavage means to the reaction mixture.
The invention is not limited by the means employed for the detection of the non-target cleavage products. For example, the products generated by the cleavage reaction (i.e., the non-target cleavage products) may be detected by their separation of the reaction products on agarose or polyacrylamide gels and staining with ethidium bromide. Other non-gel-based detection methods are provided herein.
It is contemplated that the oligonucleotides may be labelled. Thus, if the cleavage reaction employs a first oligonucleotide containing a label, detection of the non-target cleavage products may comprise detection of the label. The invention is not limited by the nature of the label chosen, including, but not limited to, labels which comprise a dye or a radionucleotide (e.g., 32P), fluorescein moiety, a biotin moiety, luminogenic, fluorogenic, phosphorescent, or fluors in combination with moieties that can suppress emission by fluorescence energy transfer (FET). Numerous methods are available for the detection of nucleic acids containing any of the above-listed labels. For example, biotin-labeled oligonucleotide(s) may be detected using non-isotopic detection methods which employ streptavidin-alkaline phosphatase conjugates. Fluorescein-labelled oligonucleotide(s) may be detected using a fluorescein-imager.
It is also contemplated that labelled oligonucleotides (cleaved or uncleaved) may be separated by means other than electrophoresis. For example, biotin-labelled oligonucleotides may be separated from nucleic acid present in the reaction mixture using para-magnetic or magnetic beads, or particles which are coated with avidin (or streptavidin). In this manner, the biotinylated oligonucleotide/avidin-magnetic bead complex can be physically separated from the other components in the mixture by exposing the complexes to a magnetic field. Additionally, the signal from the cleaved oligonucleotides may be resolved from that of the uncleaved oligonucleotides without physical separation. For example, a change in size, and therefore rate of rotation in solution of fluorescent molecules can be detected by fluorescence polarization analysis.
In a preferred embodiment, the reaction conditions comprise a cleavage reaction temperature which is less than the melting temperature of the first oligonucleotide and greater than the melting temperature of the 3xe2x80x2 portion of the first oligonucleotide. In a particularly preferred embodiment, the reaction temperature is between approximately 40-65xc2x0 C. It is contemplated that the reaction temperature at which the cleavage reaction occurs be selected with regard to the guidelines provided in the Description of the Invention.
The invention is not limited by the nature of the oligonucleotides employed. Using a target RNA, the oligonucleotides may comprise DNA, RNA or an oligonucleotide comprising a mixture of RNA and DNA.
The invention also contemplates the use of a second oligonucleotide (i.e., the upstream oligonucleotide) which comprises a functional group (e.g., a 5xe2x80x2 peptide region) which prevents the dissociation of the 5xe2x80x2 portion of the second oligonucleotide from the first region of the target RNA. When such a functional group is present on the second oligonucleotide, the interaction between the 3xe2x80x2 portion of the second oligonucleotide and the first region of the target RNA may be destabilized (i.e., designed to have a lower local melting temperature) through the use of A-T (or A-U) rich sequences, base analogs that form fewer hydrogen bonds (e.g., dG-dU pairs) or through the use of phosphorothioate backbones, in order to allow the 5xe2x80x2 region of the first oligonucleotide to compete successfully for hybridization.
In a preferred embodiment, the cleavage means comprises a thermostable 5xe2x80x2 nuclease. The thermostable 5xe2x80x2 nuclease may have a portion of the amino acid sequence that is homologous to a portion of the amino acid sequence of a thermostable DNA polymerase derived from a thermophilic organism. It is contemplated that thermophilic organisms will be selected from such species as those within the genus Thermus, including, but not limited to Thermus aquaticus, Thermus flavus and Thermus thermophilus. Preferred nucleases are encoded by DNA sequences selected from the group consisting of SEQ ID NOS:1-3, 9, 10, 12, 21, 30 and 31.
In one embodiment, the present invention contemplates a DNA sequence encoding a DNA polymerase altered in sequence (i.e., a xe2x80x9cmutantxe2x80x9d DNA polymerase) relative to the native sequence, such that it exhibits altered DNA synthetic activity from that of the native (i.e., xe2x80x9cwild typexe2x80x9d) DNA polymerase. With regard to the polymerase, a complete absence of synthesis is not required. However, it is desired that cleavage reactions occur in the absence of polymerase activity at a level that interferes with the method. It is preferred that the encoded DNA polymerase is altered such that it exhibits reduced synthetic activity from that of the native DNA polymerase. In this manner, the enzymes of the invention are nucleases and are capable of cleaving nucleic acids in a structure-specific manner. Importantly, the nucleases of the present invention are capable of cleaving cleavage structures to create discrete cleavage products.
The present invention utilizes such enzymes in methods for detection and characterization of nucleic acid sequences and sequence changes. The present invention also relates to means for cleaving a nucleic acid cleavage structure in a site-specific manner. Nuclease activity is used to screen for known and unknown mutations, including single base changes, in nucleic acids.
The invention is not limited to use of oligonucleotides which are completely complementary to their cognate target sequences. In one embodiment, both the first and second oligonucleotides are completely complementary to the target RNA. In another embodiment, the first oligonucleotide is partially complementary to the target RNA. In yet another embodiment, the second oligonucleotide is partially complementary to the target RNA. In yet another embodiment, both the first and the second oligonucleotide are partially complementary to the target RNA.
In a preferred embodiment, the methods of the invention employ a source of target RNA which comprises a sample selected from the group including, but not limited to blood, saliva, cerebral spinal fluid, pleural fluid, milk, lymph, sputum and semen.
In a preferred embodiment, the method employs reaction conditions which comprise providing a source of divalent cations. In a particularly preferred embodiment, the divalent cation is selected from the group comprising Mn2+ and Mg2+ ions.
The novel detection methods of the invention may be employed for the detection of target RNAs including, but not limited to, target RNAs comprising wild type and mutant alleles of genes, including genes from humans or other animals that are or may be associated with disease or cancer. In addition, the methods of the invention may be used for the detection of and/or identification of strains of microorganisms, including bacteria, fungi, protozoa, ciliates and viruses (and in particular for the detection and identification of RNA viruses, such as HCV).
The present invention further provides a method of separating nucleic acid molecules, comprising: a) providing: i) a charge-balanced oligonucleotide and ii) a reactant; b) mixing the charge-balanced oligonucleotide with the reactant to create a reaction mixture under conditions such that a charge-unbalanced oligonucleotide is produced; and c) separating the charge-unbalanced oligonucleotide from the reaction mixture.
The method of the present invention is not limited by the nature of the reactant employed. In a preferred embodiment the reactant comprises a cleavage means. In a particularly preferred embodiment, the cleavage means is an endonuclease. In another embodiment, the cleavage means is an exonuclease. In a still further embodiment, the reactant comprises a polymerization means. In another embodiment, the reactant comprises a ligation means.
In a preferred embodiment, the charge-balanced oligonucleotide comprises a label. The invention is not limited by the nature of the label chosen, including, but not limited to, labels which comprise a dye or a radionucleotide (e.g., 32P), fluorescein moiety, a biotin moiety, luminogenic, fluorogenic, phosphorescent, or fluors in combination with moieties that can suppress emission by fluorescence energy transfer (FET). The label may be a charged moeity or alternatively may be a charge neutral moeity.
In another preferred embodiment, the charge-balanced oligonucleotide comprises one or more phosphonate groups. In a preferred embodiment, the phosphonate group is a methylphosphonate group.
In one embodiment, the charge-balanced oligonucleotide has a net neutral charge and the charge-unbalanced oligonucleotide has a net positive charge. Alternatively, the charge-balanced oligonucleotide has a net neutral charge and the charge-unbalanced oligonucleotide has a net negative charge. In yet another alternative embodiment, the charge-balanced oligonucleotide has a net negative charge and the charge-unbalanced oligonucleotide has a net positive charge. In another embodiment, the charge-balanced oligonucleotide has a net negative charge and the charge-unbalanced oligonucleotide has a net neutral charge. In another preferred embodiment, the charge-balanced oligonucleotide has a net positive charge and the charge-unbalanced oligonucleotide has a net neutral charge. Still further, the charge-balanced oligonucleotide has a net positive charge and the charge-unbalanced oligonucleotide has a net negative charge.
In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more positively charged adducts. In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more positively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net positive charge. In another preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more positively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net neutral charge. Still further, the charge-balanced oligonucleotide comprises DNA containing one or more positively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net negative charge.
In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more negatively charged adducts (e.g., negatively charged amino acids). Examples of negative charged adducts include negatively charged amino acids (e.g., aspartate and glutamate). In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more negatively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net negative charge. In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more negatively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net neutral charge. In a preferred embodiment, the charge-balanced oligonucleotide comprises DNA containing one or more negatively charged adducts and the cleavage means removes one or more nucleotides from the charge-balanced oligonucleotide to produce the charge-unbalanced oligonucleotide, wherein the charge-unbalanced oligonucleotide has a net negative charge.
The present invention is not limited by the nature of the positively charged adduct(s) employed. In a preferred embodiment, the positively charged adducts are selected from the group consisting of indodicarbocyanine dye amidites (e.g., Cy3 and Cy5), amino-substituted nucleotides, ethidium bromide, ethidium homodimer, (1,3-propanediamino)propidium, (diethylenetriamino)propidium, thiazole orange, (N-Nxe2x80x2-tetramethyl-1,3-propanediamino)propyl thiazole orange, (N-Nxe2x80x2-tetramethyl-1,2-ethanediamino)propyl thiazole orange, thiazole orange-thiazole orange homodimer (TOTO), thiazole orande-thiazole blue heterodimer (TOTAB), thiazole orange-ethidium heterodimer 1 (TOED1), thiazole orange-ethidium heterodimer 2 (TOED2), florescien-ethidium heterodimer (FED) and positively charged amino acids.
In another preferred embodiment, the separating step comprises subjecting the reaction mixture to an electrical field comprising a positive pole and a negative pole under conditions such that the charge-unbalanced oligonucleotide migrates toward the positive pole (i.e., electrode). In another embodiment, the separating step comprises subjecting the reaction mixture to an electrical field comprising a positive pole and a negative pole under conditions such that the charge-unbalanced oligonucleotide migrates toward the negative pole.
In still further embodiment, the method of the present invention further comprises detecting the presence of the separated charge-unbalanced oligonucleotide. The present invetion is not limited by the detection method employed; the method of detection chosen will vary depending on the nature of the label employed (if one is employed).
The present invention further comprises a method of detecting cleaved nucleic molecules, comprising: a) providing: i) a homogeneous plurality of charge-balanced oligonucleotides; ii) a sample suspected of containing a target nucleic acid having a sequence comprising a first region complementary to said charge-balanced oligonucleotide; iii) a cleavage means; and iv) a reaction vessel; b) adding to said vessel, in any order, the sample, the charge-balanced oligonucleotides and the cleavage means to create a reaction mixture under conditions such that a portion of the charge-balanced oligonucleotides binds to the complementary target nucleic acid to create a bound (i.e., annealed) population, and such that the cleavage means cleaves at least a portion of said bound population of charge-balanced oligonucleotides to produce a population of unbound, charge-unbalanced oligonucleotides; and c) separating the unbound, charge-unbalanced oligonucleotides from the reaction mixture.
In a preferred embodiment, the method further comprises providing a homogeneous plurality of oligonucleotides complementary to a second region of the target nucleic acid, wherein the oligonucleotides are capable of binding to the target nucleic acid upstream of the charge-balanced oligonucleotides. In another preferred embodiment, the first and the second region of the target nucleic acid share a region of overlap.
The invention is not limited by the nature of the clevage means employed. In one embodiment, the cleavage means comprises a thermostable 5xe2x80x2 nuclease. In a preferred embodiment, a portion of the amino acid sequence of the 5xe2x80x2 nuclease is homologous to a portion of the amino acid sequence of a thermostable DNA polymerase derived from a thermophilic organism. In a preferred embodiment, the organism is selected from the group consisting of Thermus aquaticus, Thermus flavus and Thermus thermophilus. In another preferred embodiment, the nuclease is encoded by a DNA sequence selected from the group consisting of SEQ ID NOS:1-3, 9, 10, 12, 21, 30 and 31.
The invention is not limited by the nature of the target nucleic acid. The target nucleic acid may comprise single-stranded DNA, double-stranded DNA or RNA. In a preferred embodiment, the target nucleic acid comprises double-stranded DNA and prior to the addition of the cleavage means the reaction mixture is treated such that the double-stranded DNA is rendered substantially single-stranded preferably by increasing the temperature.
The invention further provides a method of separating nucleic acid molecules, comprising: a) modifying an oligonucleotide so as to produce a charge-balanced oligonucleotide; b) providing: i) a said charge-balanced oligonucleotide and ii) a reactant; c) mixing said charge-balanced oligonucleotide with said reactant to create a reaction mixture under conditions such that a charge-unbalanced oligonucleotide is produced; and d) separating said charge-unbalanced oligonucleotide from said reaction mixture.
The invention is not limited by the nature of the modification. In a preferred embodiment, the modifying step comprises the covalent attachment of a positively charged adduct to one or bases of the oligonucleotide. In another preferred embodiment, the modifying step comprises the covalent attachment of a negatively charged adduct to one or bases of the oligonucleotide. In a still further embodiment, the modifying comprises the incorporation of one or more amino-substituted bases during synthesis of the oligonucleotide. In another embodiment, the modifying comprises the incorporation of one or more phosphonate groups during synthesis of said oligonucleotide. In a preferred embodiment, the phosphonate group is a methylphosphonate group.
The invention further provides a method of treating a nucleic acid molecule, comprising: a) providing: i) a charge-balanced oligonucleotide and ii) a reactant; b) mixing said charge-balanced oligonucleotide with said reactant to create a reaction mixture under conditions such that a charge-unbalanced oligonucleotide is produced.
The invention further provides a method of treating a nucleic acid molecule, comprising: a) modifying an oligonucleotide so as to produce a charge-balanced oligonucleotide; b) providing: i) said charge-balanced oligonucleotide and ii) a reactant; c) mixing the charge-balanced oligonucleotide with the reactant to create a reaction mixture under conditions such that a charge-unbalanced oligonucleotide is produced.